1. Field of the Invention
The present invention relates to a display unit for flashlight photography and more particularly to such a display unit for providing a visual display of information relating to the object distance at which a proper exposure is obtainable by flashlight photographing operation.
2. Description of Prior Art
In a flashlight emission device cooperating with a camera for flashlight photography, the maximum quantity of emitted light from the flash discharge tube is generally limited by various factors such as the charge voltage of the main condenser and the adjustable range of flashlight illumination by the optical member for directing the emitted light from the discharge tube toward the object field. In an electronic flashlight emission device as disclosed in U.S. Pat. No. 4,210,849, only a certain limited portion of the charged energy on the main condenser is used to emit flashlight. In this type of electronic flashlight emission device, the maximum quantity of emitted light is further limited as compared to other conventional flashlight emission devices in which all of the charged energy on the main condenser are used to emit flashlight. The maximum object distance for flashlight photography is, therefore, determined depending on it whether or not any proper exposure can be obtained with the maximum quantity of emitted light. On the other hand, the minimum object distance for flashlight photography is limited by the following three different factors:
The first factor is the condition of optical illumination.
When the distance from the taking lens to the object is too small, the object is not uniformily illuminated by the flashlight because of the lack of coincidence of the optical axis of the taking lens with the optical axis of the flashlight emission part. The light is highly concentrated near the optical axis of the illumination optical system of the flashlight emission part. Due to these and other known adverse phenomenons, when the object distance exceeds a lower limit, it is no longer possible to take a good picture by flashlight photography. Therefore, the minimum object distance should be selected at a distance up to which there does not occur such adverse phenomenon.
The second factor is the possible minimum quantity of emitted light which is affected by the components of the electric circuit used to control the quantity of emitted light.
As well known to those skilled in the art, to control the quantity of emitted light, there is usually used a commutation circuit including a thyristor and a commutation condenser by which circuit the discharge current from the flash discharge tube is cut off. At the cut-off of the discharge current, an emission stop signal is generated. However, at the time there is produced a commutation current from the commutation condenser and light is emitted by the commutation current at the cut-off of the discharge current. Because of this commutation current, the flashlight emission can not be stopped at once by the stop signal but there always remains always a small amount of afterglow following the generation of a stop signal. The magnitude of this afterglow depends on the charge voltage of the main condenser. Therefore, the percentage of the standing afterglow component to the quantity of the emitted flashlight increases gradually with decreasing of the quantity of emitted light. This means that the quantity of emitted light should not be decreased without limitation in order to maintain the practically desirable level of accuracy of control on the quantity of emitted light. On the other hand, the time from the emission start to the emmission stop becomes shorter with a decrease in the quantity of emitted light. In practice, signal transmission between circuits has necessarily a certain definite time lag. The accuracy of control of the quantity of emitted light is affected by this steady time lag of signal transmission. When the emission time is very short, the effect of time lag on the control accuracy becomes much more remarkable. In this sense, the possible minimum quantity of emitted light is limited also by such signal time lag. Therefore, the minimum object distance for flashlight photography is limited also by this limitation imposed on the minimum quantity of emitted light.
The third factor is the change in spectral characteristics of the flashlight.
The spectral characteristics of the flashlight is adversely changed with the decrease of flashlight emission time. This change of spectral characteristics is destructive to the color balance and, therefore, it has an adverse effect on picture-taking using a color film. To obtain a photographically acceptable exposure it is required to impose a limitation to the minimum quantity of emitted light also in this point of view. Consequently, the minimum object distance is limited also by this third factor.